This invention relates to digital data transmission systems, and more particularly, to a circuit for immediately establishing a phase relationship of a serial digital data stream wherein a receiving and transmitting circuit each have independent clocking circuits.
In transmitting serial digital data, it is necessary to provide a series of clock pulses in addition to the data pulses in order to synchronize the coding of the transmitted serial digital data at a receiving terminal or the receiver. In prior-known transmitting schemes, two transmission lines or paths, or two channels were required for this purpose, one for the data signal and one for the clocking signal. In attempting to avoid the requirement for a dual transmission path (or two channels), self-clocking codes evolved as a result of combining the clock and data signals. The Manchester code is one such self-clocking code. Manchester encoders accept clock and data and combine them into a single output which may be transmitted by a single transmission line to the receiving terminal. When the self-clocking data (i.e., the serial digital data stream) arrives at the receiving terminal, it is processed by a decoder which extracts separately both data and clock from the input self-clocking coded signal. Generally, these prior-known transmitting schemes require several bit-times for establishing the phase relationship between the serial digital data stream and the timing of the receiver. Sometimes this is achieved by a preamble defined in a protocol.
Thus there is a need for providing an apparatus which can essentially instantaneously change phase relationship to correspond to the phase relationship of the incoming data.